Time-varying isobaric surface reconstruction and path planning for robotic grinding of weak-stiffness workpieces

Severe deformations and vibration usually occur when grinding the weak-stiffness workpieces, then fluctuate the grinding force and damage the surface. In this paper, the time-varying isobaric surface (TVIS) is defined as a virtual surface to generate constant force during robotic grinding. Based on it, a novel robotic grinding method, including contact trial and surface reconstruction, is proposed. In the contact trial process, the robot actively samples the deformation and stiffness of contact point with a force sensor. Then, a TVIS mesh is constructed to replace the original geometry of the workpiece, which is utilized for grinding path planning. Experiments have been conducted to verify the feasibility of this method. The result shows that the proposed method can achieve constant grinding force and is robust to the types of workpieces and the processing techniques. Furthermore, it is considered as an intelligent method for customized robotic machining of the weak-stiffness workpieces.     

机器人砂带磨削系统作业精度分析与误差补偿

为了更好地反映及提高工业机器人砂带磨削系统的整体性能,通过分析机器人应用系统的特点,详细 描述了工业机器人应用系统“作业精度”的含义及衡量标准．在此基础上,推导了机器人砂带磨削系统作业精度模 型,设计了机器人砂带磨削系统作业误差测量工具及校准系统,建立了实际的机器人砂带磨削系统．通过实际的机 器人磨削实验验证了方法的有效性．     

A new force-depth model for robotic abrasive belt grinding and confirmation by grinding of the Inconel 718 alloy

Robotic abrasive belt grinding has been successfully applied to the grinding and polishing of aerospace parts. However, due to the flexible characteristics of robotic abrasive belt grinding and the time-varying characteristics of the polishing contact force, as well as the plastic and difficult-to-machine material properties of Inconel 718 alloy, it is very difficult to control the actual removal depth and force of the polished surface, which brings great challenges to robot automatic polishing. Therefore, the relationship between the grinding force and the grinding depth in the robotic abrasive belt grinding is analyzed in detail, the robot machining pose error model considering the deformation of the grinding head is established, and the Inconel 718 alloy machining experiment of the robotic abrasive belt grinding is designed. The mapping relationship between the grinding force and the grinding depth is obtained, and the grinding force ratio in the downgrinding and upgrinding mode is discussed. The experimental and theoretical comparisons results show that with the increase of the grinding depress depth, both the grinding depth and the grinding force show an irregular increasing trend, and the increasing trend of the grinding force (increases by about 344.44%–445.45%) is obviously greater than that of the grinding depth (increases by about 52.94%). When the grinding depress depth is large (greater than 3 mm), the feed direction force and the normal force appear obvious secondary pressure peaks at the beginning and end of grinding, which has not been seen in previous studies. In addition, regardless of whether it is downgrinding or upgrinding, the grinding force ratio decreases with the increase of the depress depth, and the grinding force ratio of downgrinding (average 0.668) is smaller than that of upgrinding (average 0.724). This study provides a reference for robotic abrasive belt grinding, and the surface quality of Inconel 718 alloy of robotic abrasive belt grinding can be further improved through the optimization of force and depth.     

Accurate robotic belt grinding of workpieces with complex geometries using relative calibration techniques

Robotic belt grinding operations are performed by mounting a workpiece to the end effector and commanding it to move along a trajectory while maintaining contact with the belt grinding wheel. A constant contact force throughout the grinding process is necessary to provide a smooth finish on the workpiece, but it is difficult to maintain this force due to a multitude of installation, manipulation, and calibration errors. The following describes a novel methodology for robotic belt grinding, which primarily focuses on system calibration and force control to improve grinding performance. The overall theory is described and experimental results of turbine blade grinding for each step of the methodology are shown.     

Generic development methodology for flexible robotic pick-and-place workcells based on Digital Twin

Together with the trends of mass personalization, flexible robotic applications become more and more popular. Although conventional robotic automation of workpiece manipulation seems to be solved, advanced tasks still need great amount of effort to be reached. In most cases, on-site robot programming methods, which are intuitive and easy to use, are not applicable in flexible scenarios. On the other hand, the application of offline programming methods requires careful modeling and planning. Consequently, this paper proposes a generalized development methodology for flexible robotic pick-and-place workcells, in order to provide guidance and thus facilitate the development process. The methodology is based on the Digital Twin (DT) concept, which allows the iterative refinement of the workcell both in the digital and in the physical space. The goal is to speed up the overall commissioning (or reconfiguration) process and reduce the amount of work in the physical workcell. This can be achieved by digitizing and automating the development, and maintaining sufficient twin closeness. With that, the operation of the digital model can be accurately realized in the physical workcell. The methodology is presented through a semi-structured pick-and-place task, realized in an experimental robotic workcell together with a reconfiguration scenario.     

Identification of effective engineering methods for controlling handheld workpiece vibration in grinding processes

The objective of this study is to identify effective engineering methods for controlling handheld workpiece vibration during grinding processes. Prolonged and intensive exposures to such vibration can cause hand-arm vibration syndrome among workers performing workpiece grinding, but how to effectively control these exposures remains an important issue. This study developed a methodology for performing their analyses and evaluations based on a model of the entire grinding machine-workpiece-hand-arm system. The model can simulate the vibration responses of a workpiece held in the worker's hands and pressed against a grinding wheel in order to shape the workpiece in the major frequency range of concern (6.3–1600 Hz). The methodology was evaluated using available experimental data. The results suggest that the methodology is acceptable for these analyses and evaluations. The results also suggest that the workpiece vibration resulting from the machine vibration generally depends on two mechanisms or pathways: (1) the direct vibration transmission from the grinding machine; and (2) the indirect transmission that depends on both the machine vibration transmission to the workpiece and the interface excitation transformation to the workpiece vibration. The methodology was applied to explore and/or analyze various engineering methods for controlling workpiece vibrations. The modeling results suggest that while these intervention methods have different advantages and limitations, some of their combinations can effectively reduce the vibration exposures of grinding workers. These findings can be used as guidance for selecting and developing more effective technologies to control handheld workpiece vibration exposures.     

Hybrid active/passive force control strategy for grinding marks suppression and profile accuracy enhancement in robotic belt grinding of turbine blade

Grinding marks and traces, as well as the over- and under-cutting phenomenon are the severe challenges in robotic abrasive belt grinding of turbine blades and it greatly limits the further application of robotic machining technology in the thin-walled blade fields. In the paper, an active force control method consisting of force/positon and PI/PD controller based on six-dimensional force/torque sensor is introduced to eliminate the grinding marks and traces, and a passive force control method including PID controller based on one-dimensional force sensor is proposed to reduce the over- and under-cutting phenomenon in robotic machining system. Then the Kalman filter information fusion methodology is adopted to combine the active and passive force control methods which could improve the controlled force accuracy and efficiency, as well as avoid the control interference. Finally both the test workpiece and turbine blade are employed to examine and verify the reliability and practicality of the proposed hybrid force control method by achieving the desired surface quality and higher profile precision.     

A new calibration method for a dynamic coordinate system in a robotic blade grinding and polishing system based on the six-point limit principle

Automatic robotic grinding and polishing systems have become a developing trend in aerospace parts manufacturing. In a robotic blade grinding and polishing system (RBGPS), the automatic and precise calibration of the dynamic workpiece coordinate frame is the most important process. In this research, a new method that introduces the concept of six-point positioning into the dynamic workpiece coordinate frame calibration process is proposed using a point laser displacement sensor (PLDS). The static coordinate frame calibration process is conducted based on a robot flange and force sensor. The results indicate that the new method can achieve a higher precision calibration result and has improved operational efficiency and cost. Finally, its practicality is verified in the BRGPS, and the results indicate that the polished blade surface after using the new method has good consistency.     

A Workcell for the Development of Robot-Assisted Surgical Procedures

This paper describes a robotic workstation for the development of new robot-assisted surgical procedures. This work is motivated by the difficulties and cost associated to the development of surgical robots, often requiring large investments and several re-designs which limit wider use of this technology. The approach presented here consists of using a general purpose robotic workcell to develop the hardware and the surgical aspects of new robot-based surgical systems, before committing to a completely new system design. The workcell is based on a clean room PUMA 260 manipulator, suitably enhanced to expand and improve its capabilities, and on a vision-based operator interface. Two new robot-assisted surgical procedures have been developed and tested using this set-up: percutaneous discectomy and knee osteoctomy. By using the robotic workcell, engineers and surgeons are able to define many aspects of the two procedures, such as surgical gestures, workspace of the robot, and calibration procedures, without incurring a large, up-front investment. First, the article describes the configuration of the workcell, the enhancements to the PUMA manipulator and the surgical procedures developed with this setup. Then the results of the tests and the lessons learned using the workcell are discussed in some detail.     

Robotic grinding of complex components: A step towards efficient and intelligent machining – challenges,solutions, and applications

Robotic grinding is considered as an alternative towards the efficient and intelligent machining of complex components by virtue of its flexibility, intelligence and cost efficiency, particularly in comparison with the current mainstream manufacturing modes. The advances in robotic grinding during the past one to two decades present two extremes: one aims to solve the problem of precision machining of small-scale complex surfaces, the other emphasizes on the efficient machining of large-scale complex structures. To achieve efficient and intelligent grinding of these two different types of complex components, researchers have attempted to conquer key technologies and develop relevant machining system. The aim of this paper is to present a systematic, critical, and comprehensively review of all aspects of robotic grinding of complex components, especially focusing on three research objectives.For the first research objective, the problems and challenges arising out of robotic grinding of complex components are identified from three aspects of accuracy control, compliance control and cooperative control, and their impact on the machined workpiece geometrical accuracy, surface integrity and machining efficiency are also identified. For the second aim of this review, the relevant research work in the field of robotic grinding till the date are organized, and the various strategies and alternative solutions to overcome the challenges are provided. The research perspectives are concentrated primarily on the high-precision online measurement, grinding allowance control, constant contact force control, and surface integrity from robotic grinding, thereby potentially constructing the integration of “measurement – manipulation – machining” for the robotic grinding system. For the third objective, typical applications of this research work to implement successful robotic grinding of turbine blades and large-scale complex structures are discussed. Some research interests for future work to promote robotic grinding of complex components towards more intelligent and efficient in practical applications are also suggested.     

Enhancement and evaluation in path accuracy of industrial robot for complex surface grinding

Lower path accuracy is an obstacle to the application of industrial robots in intelligent and precision grinding complex surfaces. This paper proposes a novel path accuracy enhancement strategy and different evaluation methods for a six-degree-of-freedom industrial robot FANUC M710ic/50 used for grinding an aero-engine blade. Six groups of theoretical tool paths individually planned on this complex surface were obtained using the iso-parametric method and the constant chord height method. Then the actual paths of the robot were dynamically recorded by a laser tracker with a high frequency. A revised Levenberg-Marquardt and Differential Evolution hybrid algorithm was proposed to improve the absolute robotic positioning accuracy by considering the average curvature variation rate, the arc length and the number of cutter contact points on planning paths. The results showed that the maximum positioning error had been drastically reduced from 0.792 mm to 0.027 mm. Based on the redefinition of robotic path accuracy, including position accuracy and shape accuracy in this work, the methods MP-TLD, BP-TPD and MP-TID were proposed to evaluate the enhanced path accuracy. The evaluation results showed that the different path planning methods have almost little effect on path accuracy. Furthermore, the maximum path deviation evaluated by the MP-TLD method was reduced from 0.378 mm to 0.044 mm, evaluated by the BP-TPD method was reduced from 0.374 mm to 0.029 mm, and evaluated by the MP-TID method was reduced from 0.205 mm to 0.026 mm. It is concluded that these evaluation methods are basically valid and the average path accuracy value is about 0.035 mm, for present complex surface grinding with this typical industrial robot. Finally, the robotic grinding experiments of titanium alloy blades are conducted to further validate the effectiveness of the proposed method.     

Dynamic response of a workpiece in turning with continuously varying speed

In order to study the influence of workpiece speed variation on the dynamic response of a workpiece in turning operations, this paper develops a finite element model to describe the lateral vibration of a rotating Euler-beam, in which both axial force and damping effect are included and the workpiece speed changes continuously and periodically. A simplified cutting force model which depends on the feed-rate and workpiece deflection is used along with the proposed finite element model. To solve the obtained finite element equations of a workpiece in turning, the Newmark integration scheme is employed to further discretize the time domain. At each time step the resulting nonlinear equations are solved iteratively. The numerical results show the influence of the workpiece speed variation frequency and amplitude on the dynamic response of turning operations. Also, the suppression of self-excited vibration in turning by changing the workpiece speed periodically is demonstrated by using the present analytical model.     

Investigating the effects of vibration method on ultrasonic-assisted drilling of Al/SiCp metal matrix composites

Preciseness and finished surface quality are the significant factors of final products, especially in a number of drilling processes. Burr is usually considered a negative outcome in assembly procedures. One way to reduce or remove burr and improve surface roughness in metal cutting is to employ ultrasonic vibration in the drilling process. In this paper, the effects of ultrasonic vibration on burr size reduction, drilling force and surface roughness with two different vibration systems are investigated. To this end, two vibration structures were built, one to excite the workpiece (the workpiece vibration system) and the other to vibrate the tool (the tool vibration system). Besides, the effects of amplitude, feed rate, cutting speed, and SiC particle content on the drilling process of Al/SiCp metal matrix composites are studied. In all tests TiN-coated HSS drill tools with a diameter of 5 mm were utilized for drilling. Based on the attained results, it was demonstrated that suitable ultrasonic vibration reduced burr height, drilling force and surface roughness more so than conventional drilling. Meanwhile, in the workpiece vibration system, enhanced surface roughness and higher drilling force were obtained as compared to the tool vibration system.     

Dynamic control of a robot manipulator for high-performance conveyor tracking

A new robot control scheme for the specific application to conveyor tracking is presented. To improve the performance of conveyor tracking, the robot arm dynamics is incorporated into the control scheme. The tracking problem for the workpiece on a variable-speed conveyor is formulated as a stochastic optimal tracking problem with specified criteria. Dividing the conveyor speed into the nominal term and the perturbed term, a two-stage control strategy is employed to cope with the nonlinearity and uncertainty of the robot-conveyor system. Simulation results are given to verify the good tracking performance with fast cycle time and high accuracy obtained in a robotic workcell. © 1995 John Wiley & Sons, Inc.     

A database design for the runtime environment of a robotic workcell

Intelligent robotic workcell activities have come to require a database framework for arranging, storing and accessing information in real-time about the workcell environment in a standard way. After a brief introduction to the theory of databases (DB), we examine the general topic of robotic workcells, identify characteristics typical of robotic applications, and then present a survey of DB-related work in the robotics domain. We then construct a set of design constraints based on our analysis of robotic applications, and describe a suitable software architecture. The paper concludes with a discussion of experience gained with two generations of DB implementations.     

The OR dataflow architecture for a machine embedded control system

This paper presents a new architecture for embedded systems and describes an appropriate method for programming a control system. A grinding machine control system was built and an experimental verification of the theoretical approach was performed. The efficiency of this novel system was compared with the conventional control systems by grinding a workpiece up to the stringent quality requirements. The superior performance of the OR dataflow control system lead to the encouraging conclusions presented in this paper.     

Impedance model force control using a neural network-based effective stiffness estimator for a desktop NC machine tool

In manufacturing industries of metallic molds, various NC machine tools are used. A desktop NC machine tool with compliance control capability has been already proposed to automatically cope with the finishing process of an LED lens mold. The NC machine tool can control the polishing force acting between an abrasive tool and a workpiece, where the force control method developed is an impedance model force control. The most important gain, which gives a large influence to the stability, is the desired damping of the impedance model. Ideally, the desired damping is calculated from the critical damping condition of the force control system in consideration of the effective stiffness. The effective stiffness means the total stiffness including the characteristics composed of the NC machine tool itself, force sensor, tool attachment, abrasive tool, workpiece, zig and floor. One of the serious problems is that the effective stiffness of the NC machine tool has undesirable nonlinearity, so that it may destroy the stability of the force control system. In this paper, a systematic tuning method of the desired damping in the control system is considered by using neural networks, where the neural networks acquire the nonlinearity of effective stiffness. It is confirmed that the impedance model force controller with the neural network-based (NN-based) stiffness estimator allows the NC machine tool to achieve a high quality finished surface of an LED lens mold with a diameter of 3.6 mm.     

Feasibility study of robotic fibre placement on intersecting multi-axial revolution surfaces

In this paper, the first steps towards using a robotic workcell for the automated fibre placement (AFP) manufacturing of Y-shaped tubes are proposed. The proposed workcell is constituted of a standard serial manipulator holding the fibre placement toolhead combined to a rotary table on which the part where the fibres must be laid out is attached. The investigations carried out in this work explore the feasibility of this setup and more precisely the path planning aspect. To this aim two novel path planning algorithms are presented generalizing the techniques proposed in the literature for open-contoured and cylindrical surfaces. In the first, the maximal geodesic curvature typically allowed in AFP is disregarded to generate continuous paths with a constant placement angle on the branches without any gaps or overlaps. Subsequently, a second algorithm, taking into account this curvature constraint, is presented. These algorithms were implemented in a software using the MATLAB™ suite. Finally, an algorithm to optimize the motion of the robotic system is presented and simulations are discussed.     

Measurement error analysis and accuracy enhancement of 2D vision system for robotic drilling

Robotic drilling for aircraft structures demands higher accuracy on industrial robots than their traditional applications. Positioning error measurement and compensation based on 2D vision system is a cost-effective way to improve the positioning accuracy in robotic drilling. In this paper, we first discuss the principle of error measurement and compensation with a 2D vision system for robotic drilling and the determination of tool center point of the vision system so that the Abbe errors are eliminated in the measurement process. Measurement errors due to nonideal measurement conditions, i.e. nonperpendicularity of the camera optical axis to the workpiece surface and incorrect object distance, are mathematically modeled and experimentally verified. A method utilizing four laser displacement sensors is proposed to ensure perpendicularity of the camera optical axis to the workpiece surface and correct object distance in the measurement process, and hence to achieve high accuracy in 2D vision-based measurement. Experiments performed on a robotic drilling system show that the 2D vision system can achieve an accuracy of approximately 0.1 mm with the proposed method.